Choosing spear for dry barrel gun

Diving Gecko said:

So, the Evo-Air has a bigger "internal volume" of the outer muzzle or a bigger surface area of exposed O-ring which lets more pressure on to the O-ring - is that correctly understood? I did come across a mentioning on an Italian board stating that at depth, it still functioned, so someone must have raised those issues as well. But I don't think they performed any elaborate tests to show if there was any/much loss of power. I think, they just stated that at 30 and 40(?) meters it still functioned. (All these was though Google Translate).

I am really liking the LG Sub reels and their pressure gauge.... I had been looking at Omer Match reels with the pneumatic adapter. These seem very much like the Omers but perhaps sturdier, better constructed.

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O-ring is 7 x 3 mm, cross section exposed to water pressure is about 1.15 cm2. On 30 m depth it is 4 x 1.15 = 4.6 kgf (4, not 3 kgf, because there is vacuum in a gun). This is a force pushing O-ring in to the gun. I do not believe it would be "movable" under this force. Pete, am I wrong?

Increased friction is related to the width of the seal contact on the shaft and how many seals there are, but most vacuum barrel guns seem to use only one vacuum seal in the muzzle that actually contacts the spear running surface. Differential pressure across a muzzle seal for vacuum barrel use is very low compared to say the seal on a piston separating ambient pressure from gun chamber pressures of 20 atm. and upwards (for vacuum barrel guns you add 1 atm. as the piston seal faces a near vacuum in the inner barrel). The stationary muzzle seal on the RPS-3 hydropneumatic gun has to contend with around 60 atm, but there it holds water under pressure inside the inner barrel tube rather than keeping water out. The muzzle seal used there is much wider than an "O" ring and has more capacity to handle accumulated scratch damage to the seal surface, whereas an "O" ring will fail with a small cut or knick running across the width of the contact patch which is a very thin band (unfortunately the RPS-3 rubber seals broke up due to their poor manufacture, negating any durability advantages of having a wider contact patch). That contact patch only becomes wider as high pressure distorts an "O" ring and extrudes it into the direction of low pressure, but vacuum barrel pressure differential at the muzzle will probably reach only 4 Bar (at 30 metres of depth, 3 plus 1 for the vacuum) so the "O" ring distortion will be negligible in widening the contact patch compared to that on a piston "O" ring.

The use of wider contact patch seals is probably to increase their durability with resistance to small micro nicks and abrasions on their running surface all lining up and creating a continuous leak path compared with the performance of an "O" ring in that application, but the former take longer to replace as they are usually clamped into the fitting that holds them in a fully sealed condition in the gun's muzzle, whereas "O" rings are held captive in their machined to suit grooves as they have a small section width compared to the non-"O" ring seals made for this application. If you use a system that requires special seals then you are dependent on accessing those seals when your supply of spares run out, whereas "O'" rings can be found more easily, provided that they are not some "special order" size made for the manufacturer!

Hence the frictional effect on shooting performance may be less important than maintaining the gun's operation in the short to medium term, as most fish are usually shot well within the kill range of a gun, unless the user is trying to wring too much performance out of a short gun. If something makes a material difference then it is worth worrying about, so order of magnitude needs to be considered before becoming too excited about what appear to be theoretical advantages or disadvantages in some areas.

Marketing claims and reality are not always the same thing, marketing tries to build on perceptions rather than fact and misleading statements are often made which exaggerate some claimed advantages. They are after all in the business of persuading people to upgrade their guns , or buy something different.

Thanks Pete!
Here is an interesting diagram regarding O-ring compression. If I am not wrong, to compress (10 - 20 %) O-ring of cross section 3 mm on 7 mm shaft, force of about 3.3 kgf is necessary. (0.7 cm * 3.14 * 15 N/cm /9.81). If friction coefficient would be 0.1 (I dont know the exact value) than friction between shaft and O-ring is about 0.33 kgf. This is what I have in Tomba or Karayo in Tovarich. This friction is independent on water depth.
In case of movable O-ring, if the cone angle is 5 deg (I don't know the exact value), to this force of 3.3 kgf should be added (on 30 m depth) 4.6 kgf/sin 5 = 52 kgf. Total friction would be (52 + 3.3)*0.1 = 5.53 kgf. In this case friction is 5.53 kgf, while force to O-ring is 4.6 kgf so O-ring would be pushed out from its sealing position and really become movable. But if friction would be less than the force pushing O-ring into the muzzle, total friction would be higher than in case of Tomba or Tovarich.




Another question, is there really no friction after movimg the O-ring in front position? I think it is possible. That would mean that there is no compression of O-ring. No compression means no friction. But what if before loading the shaft, the O-ring would not be pushed to its back position? Water will enter the barrel passing by O-ring.

I hate muzzle heavy guns!

100cm x 6.5mm 110cm x 7mm

foxfish said:

I love the short spears in my 90 as it flies like a dart & obviously makes loading easier than a longer spear however I do use 100cm & 110cm spears. The 110 is 7mm & the 100 is 6.5 so there is quite a wieght difference in the spears. I much prefer the 100cm for my use.
All my air gun spears are double barb Omer black SS, never seen any rust on them or to be honest on any of the spears I own!

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Sorry for the short reply I was using my phone earlier!
I find the shorter spear best for me as the 110 definitely makes the gun muzzle heavy & that is on my custom gun with extra buoyancy!
I really dont enjoy using an unbalanced gun it stops me concentrating on what I need to be thinking about & thinking about my aching wrist instead.
I would buy the standard length spear recommended for the gun?

trromic said:

Thanks Pete!
Here is an interesting diagram regarding O-ring compression. If I am not wrong, to compress (10 - 20 %) O-ring of cross section 3 mm on 7 mm shaft, force of about 3.3 kgf is necessary. (0.7 cm * 3.14 * 15 N/cm /9.81). If friction coefficient would be 0.1 (I dont know the exact value) than friction between shaft and O-ring is about 0.33 kgf. This is what I have in Tomba or Karayo in Tovarich. This friction is independent on water depth.
In case of movable O-ring, if the cone angle is 5 deg (I don't know the exact value), to this force of 3.3 kgf should be added (on 30 m depth) 4.6 kgf/sin 5 = 52 kgf. Total friction would be (52 + 3.3)*0.1 = 5.53 kgf. In this case friction is 5.53 kgf, while force to O-ring is 4.6 kgf so O-ring would be pushed out from its sealing position and really become movable. But if friction would be less than the force pushing O-ring into the muzzle, total friction would be higher than in case of Tomba or Tovarich.




Another question, is there really no friction after movimg the O-ring in front position? I think it is possible. That would mean that there is no compression of O-ring. No compression means no friction. But what if before loading the shaft, the O-ring would not be pushed to its back position? Water will enter the barrel passing by O-ring.

Click to expand...

You need to remember that the muzzle vacuum seal "O" ring is wet on both inner and outer faces before the spear shaft goes into the gun, so the rubber ring and its seating grooves are already wet. That water lowers the friction, especially once the shaft starts moving through the seal as the moisture in the inner barrel lubricates the seal interface because a vacuum barrel gun is never truly dry inside, some water was already inside the muzzle and around the front of the piston before the shaft tail entered the gun and the muzzle seal made initial contact on the spear shaft surface. On non-"O" ring seals such as rubber packing's and square section rubber washers the seating location and ring section inside the seat should be dry as the material is squeezed up by the mechanical clamping process. Where water does lubricate the rubber ring the friction is reduced as the shaft passes transversely through the ring, unlike the situation where the shaft revolves in the ring and stays dry on one side, such as in an underwater camera case control rod. In that application the seals are greased as the intention is that water never gets under the rubber seal, lubrication being provided by the grease on the "dry side".

Regarding friction coeficient of O-rings, I saw an interesting diagram where you can see that friction coeficient might be in range of 0.001 - 1 depending on material, hardness, viscosity, relativ speed, surface smoothness ...

Being a metallurgist by trade, if you can score ona Ti rod its the best, I use it in all my spearguns, if not the 17-4 PH is the next best choice. about 17% Cr and 3-5%Ni good for resistance against Rust, PH only mean Precipitation Hardened

poildeq said:

Being a metallurgist by trade, if you can score ona Ti rod its the best, I use it in all my spearguns, if not the 17-4 PH is the next best choice. about 17% Cr and 3-5%Ni good for resistance against Rust, PH only mean Precipitation Hardened

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Ti is almost 2 x lighter than stainless steel. That means lower pressure in a gun, faster spear, but lower impact (momentum, m * v).

nice to be reading the forums again after a few months away from diving n spearing. especially the pneumatic dry barrel threads!

just a quick question as i need to get a few extra spears. i've been using the STC X-power with "freeshaft" signal spear with hole near barb, but i also have the STC slider where you tie a knot on the line about 10cm from the tail of the spear. will this be less or more drag than using plastic slider with tail? i've attached the slider for reference. appreciate any advice so i can figure out which spears will be faster and more accurate before i buy. cheers guys!

i really didn't like the tail free with mono tied in front at first because the spear would curve off, especially on longer shots. but after i added the STC slider and green ball and knot, shots were much straighter. there was a problem with STC slider that were annoying. sometimes when you shoot a fish, the slider would not go through the fish (especially when you hit a fish more lengthwise) and the slider would pull off the end of the tail. so you have to make sure the mono you are using is not too flexible and/or you have to tie the knot and green ball higher up the mono. i always try to tie the knot as close to tail as possible which makes the shaft curve off later, but you can't tie it too close to tail or it will pop off.

as far as the screw-on tail type, would the friction from the tail hitting the dry barrel muzzle slow the spear down more or would the stc slider and mono running along the shaft slow if down more?

another thing about the stc is that the spear is tight in the muzzle. i mean compared to the EVO where I guess you screw it looser to allow the shaft to be not so tight after inserting or Tomislav's muzzle which loosens the spear on the way out. i haven't used with evo or Tomislav's muzzles yet, but i really like that idea and imagine that would speed up the shaft quite a bit.